1. Field of the Invention
The present invention relates to a process for producing an alkaline paper, and in particular, to a process for producing alkaline-sized papers exhibiting an excellent sizing property and entailing no hazard to human health, in which formation of slime is substantially inhibited.
2. Description of the Related Art
Slime is viscous and heterogeneous material formed by microbiological growth at various points within a paper stock preparation system, particularly, at points such as inside wall of pipelines, chests and the like where flow rate of the paper stock is slow. Slime becomes foreign matter to paper, and forms spots on paper. These adversely affect the appearance of paper. In addition, slime causes paper breaks and soiling of press fabrics. These disturb the operation of papermaking. Therefore, various methods have been contemplated to control slime.
Among the methods, one which is thought to be the most effective is suppressing microbiological growth or sterilizing microorganisms by adding slimecides to a paper stock.
However, it is difficult nowadays to find an effective slimecide and to determine a spot of addition in paper making systems where increased recycling of white water and a shift from an acid system to an alkaline system are required.
Moreover, most slimecides are toxic and affecting harmfully a living thing, so that safety of the papers which might be contaminated by use of such chemicals must be proved.
As slimecides, organic nitrogen and sulfur containing compounds such as 5-chloro-2-methyl-4-isothiazoline-3-thione and the like, organic bromine compounds such as 2,2-dibromopropionamide and the like, and organic nitrogen compounds such as dichloroisocyanate and the like have heretofore been known. These chemicals are added to a paper stock continuously or shockwise (or in a "slug" method--where the toxicants are added fairly rapidly over a relatively short period of time in order to maintain a high concentration for a short time).
It happens often that a sufficient slime controlling effect can not be obtained in a paper making system which has shifted to alkaline and in which slimecides are added to a paper stock according to the conventional methods. In an alkaline paper making system, a considerable amount of a cationic starch which is a nutrient source for microorganisms is used and pH of the stock system is maintained within a range, 7.0-8.5, favoring microbiological growth.
A slimecide, if there any, having sufficient slime controlling effect though under that environment, may interfere with sizing performance of alkylketene dimers used in an alkaline paper making system as sizing agents, or may be one too toxic and safety of it is hardly proved.
Therefore, it has been very difficult to find appropriate slimecides and slime controlling methods.
In order to solve the aforementioned problems, the present inventors have carried out researches on slime controlling effects of a variety of slimecides to be added to an alkaline paper stock system containing a cationic starch and an alkylketene dimer, safety to man in use of the toxicants and their influence on sizing characteristics of papers made of the stock.
As a result, it has been found that specific slimecides are favorable. The researches on their use has brought to light a specific manner of addition of them which further stimulates their slime controlling effects.